Migrate to Spring Boot for deployment

Description

Use Spring Boot as wrapper rather than depending on deployment as a Tomcat WAR. Most of the core configuration has already been migrated toward using the correct Spring libraries, so it makes sense to get rid of the external Tomcat dependency. Instead, Spring Boot will start the application and use it's embedded Tomcat engine. This approach allows SiteWhere to be started as an executable WAR directly from the command line. The Gradle scripts should be updated to generate a server archive that is more streamlined since there are no Tomcat configuration dependencies.